1. Field of the invention
The present invention relates to an image forming apparatus, having a moving type developing unit carrying a plurality of developing devices, for developing an electrostatic latent image formed on a photosensitive drum by a selected developing device at a predetermined position.
2. Description of the Related Art
According to a image forming apparatus, in which a developing unit is moved, disclosed in Japanese Patent Laid-Open Publication No. 57-204567, a developing unit which moves vertically carries a plurality of developing devices. In the above apparatus, a selected developing device accommodating a color toner is stopped at a position, for performing a developing operation, opposed to a photosensitive drum. The developing device is moved between a position adjacent to a photosensitive drum at which the developing sleeve is opposed to the photosensitive drum with a predetermined distance spaced therebetween and a position, spaced a predetermined distance from the movement of the developing unit.
In this apparatus, if the developing device is not positioned with the predetermined position away from the photosensitive drum in moving the developing device toward the photosensitive drum, i.e., if the distance between the developing sleeve and the photosensitive drum is not appropriate, a normal developing operation cannot be accomplished. As a result, the following disadvantages occur: the density of a developed image is light; copy paper having no electrostatic latent image developed is discharged from the apparatus; or an image having a desired color is not developed. It is necessary to move the developing device from the position adjacent to the photosensitive drum to the position spaced from the photosensitive drum after a developing operation is completed. If at this time, the developing device is not returned to the position for allowing the movement of the developing unit due to an abnormality of a mechanism for moving the developing device, the developing device prevents the developing unit from moving vertically. An operation for forcibly moving the developing unit damages the developing device, the photosensitive drum, or members positioned in the vicinity thereof.
However, none of the conventional image forming apparatuses have countermeasures for overcoming the above disadvantage.
In order to securely stop a selected developing device at the developing position in an image forming apparatus in which the developing unit is moved, it is necessary to brake a motor for driving the developing unit at an appropriate timing. According to an apparatus in which the developing unit is locked by a locking mechanism at a desired position, it is necessary to lock the developing unit at the developing position after a selected developing unit is reliably stopped there so that a lock-caused shock does not affect a developing operation.
Various systems having such a function as described above have been proposed, for example, in U.S. Pat. No. 4,615,612 which discloses an image forming apparatus having a developing unit having a plurality of developing devices arranged in a rotary member. In this apparatus, a selected developing device is driven to the developing position to perform a development. The feature of this apparatus is that the moving speed of the developing device and/or the position of the developing device is detected. A detected value (values) is compared with a target value. The output of a motor for driving the developing unit is controlled in such a manner that the difference between the detected value and the target value is compensated. An image forming apparatus disclosed in Japanese Patent Laid-Open Publication No. 61-151564 is similar to the art of U.S. Pat. No. 4,615,612. According to the disclosure, the distance moved by the developing unit with respect to the reference position is counted by a counter. If a counted value coincides with a value predetermined according to a target value, a signal for locking the developing unit at the developing position is generated so that a locking means stops a selected developing device at the developing position. In order to permit a developing operation, the following condition must also be satisfied: A sensor detects a selected developing device at the developing position a predetermined period of time in relation to the time at which the developing device has started moving.
According to the above prior arts, the drive and stop of the developing unit and the timing for locking the developing unit are not controlled by directly detecting the current position of the developing unit, but by data obtained by calculating the current position of the developing unit based on the number of rotations of the driving motor. According to the above-described control method, if the number of rotations of the driving motor is erroneously detected or an erroneous calculation is performed, a brake cannot be applied at an appropriate timing to means for driving the developing unit. As such, the developing unit cannot be stopped at a desired position. If the number of rotations of the driving motor is erroneously detected or an erroneous calculation is performed, the developing unit cannot be locked at an appropriate timing. As a result, improper force is applied to the mechanism for locking the developing unit. As such, the locking mechanism is damaged or the developing device cannot be stopped at the predetermined position with respect to the photosensitive drum. Further, it is necessary to provide the apparatus with a detecting device such as a pulse producing disk for detecting the number of rotations of the driving motor. Thus, the apparatus is incapable of avoiding a complicated structure.